There has hitherto been known an active matrix-type liquid crystal display device provided with a thin-film transistor (TFT) as a switching element. A display portion of the active matrix-type liquid crystal display device includes a plurality of source bus lines (video signal lines), a plurality of gate bus lines (scanning signal lines), and a plurality of pixel formation portions provided corresponding respectively to intersections between the plurality of source bus lines and the plurality of gate bus lines. These pixel formation portions are arranged in a matrix form, to configure a pixel array.
FIG. 39 is a circuit diagram showing a configuration of a pixel formation portion of the conventional general active matrix-type liquid crystal display device. As shown in FIG. 39, the pixel formation portion includes: a thin-film transistor T91, whose gate electrode is connected to a gate bus line GL passing through a corresponding intersection, and whose source electrode is connected to a source bus line SL passing through the intersection; a pixel electrode 92 connected to a drain electrode of the thin-film transistor T91; a common electrode (counter electrode) COM and an auxiliary capacitance electrode CS which are provided commonly in the plurality of pixel formation portions; a liquid crystal capacitor Clc formed by the pixel electrode 92 and the common electrode COM; and an auxiliary capacitor Cstg formed by the pixel electrode 92 and the auxiliary capacitance electrode CS. Further, a pixel capacitance is formed by the liquid crystal capacitor Clc and the auxiliary capacitor Cstg. Then, a voltage indicating a pixel value is held in the pixel capacitance on the basis of a video signal that the source electrode of the thin-film transistor T91 receives from the source bus line SL when the gate electrode of the thin-film transistor T91 receives an active scanning signal from the gate bus line GL. It is to be noted that the auxiliary capacitor Cstg is not necessarily provided.
Incidentally, there has recently been apparent progress in high definition of display images in the liquid crystal display device. An Example of the high definition includes a 4K (resolution: 3840×2048) large-sized panel for television. With high-definition display images, power consumption associated with driving of the panel increases. A power attribute to the charging and discharging of the source bus line accounts for a large part of the power consumption of the panel. The power consumption attribute to the charging and discharging of the source bus is obtained by: (the number of source bus lines)×(wiring capacitance of source bus line)×(drive frequency)×(square of amplitude of video signal). Therefore, making the amplitude of the video signal smaller can effectively reduce the power consumption of the panel. As methods for making smaller the amplitude of the video signal to be applied to the source bus line, methods (first to third methods) as described below have been proposed.
A first method is a method of alternatively applying a high-level potential and a low-level potential to the common electrode, namely a method of performing alternate current (AC) drive on the common electrode. According to the first method, a polarity (a polarity with reference to a common electrode potential) of the video signal is made negative at the time of writing into the pixel capacitance in a state where a high-level potential is applied to the common electrode, and the polarity of the video signal is made positive at the time of writing into the pixel capacitance in a state where a low-level potential is applied to the common electrode. It is to be noted that the first method has been adopted to a display device of a VA (Vertical Alignment) mode, an IPS (In-Plane Switching) mode, and the like.
A second method is a method of configuring the common electrode to be separated per row, and driving each common electrode as a waveform as shown in FIG. 40. In FIG. 40, Vcom1 to Vcom4 are waveforms of the common electrode corresponding respectively to first to fourth rows. Also in the second method, as in the first method, the polarity of the video signal is made negative at the time of writing into the pixel capacitance in a state where a high-level potential is applied to the common electrode, and the polarity of the video signal is made positive at the time of writing into the pixel capacitance in a state where a low-level potential is applied to the common electrode. It is to be noted that the second method has been adopted to the display device of the IPS mode.
The third method is a method of fluctuating a potential of the auxiliary capacitance electrode after writing from the source bus line into the pixel capacitance is performed. According to the third method, after writing from the source bus line into the pixel capacitance is performed, the potential of the auxiliary capacitance electrode is changed so as to increase a voltage between the pixel electrode and the common electrode in a state where a switching element (thin-film transistor T91 of FIG. 39) called as pixel TFT or the like is turned OFF.
Further, Japanese Patent Application Laid-Open No. 2009-109600 discloses invention of a liquid crystal display device where the pixel formation portion is configured as shown in FIG. 41 and driving is performed as follows. In a first half period of one horizontal scanning period, an ON-level potential is applied to a line denoted by reference character 9 in a state where an OFF-level potential is applied to the gate bus line GL. This brings thin-film transistors T902, T903 into an ON state. As a result, a video signal potential (a potential of the source bus line SL) is applied to a node 901, and a potential of the common electrode COM is applied to a node 902. Subsequently, in a latter half of one horizontal scanning period, an ON-level potential is applied to the gate bus line GL in a state where an OFF-level potential is applied to the line denoted by reference character 9. This brings a thin-film transistor T901 into the ON state. As a result, a video signal potential is applied to the node 902. That is, the potential of the node 902 increases from the common electrode potential to the video signal potential. At this time, as the node 901 is in a floating state, a potential of the node 901 increases with increase in potential of the node 902 via a capacitor C91. In such a manner as thus described, a larger voltage is applied between the pixel electrode and the common electrode.